Electronically controlled throttle valves are contemplated for controlling the quantity of combustion air admitted to the intake manifold of internal combustion engines. These systems, typically referred to in the automotive arts as electronic throttle control systems (ETC), utilize an operator-actuated pedal position sensor which functions to transmit driver intent to an electronic actuator for positioning the throttle valve. It may be desirable to mechanically locate the throttle valve in a predetermined "default" position at times of actuator inoperativeness thereby assuring continued engine operation.
A contemplated apparatus for default positioning of the throttle valve utilizes a throttle valve having a range of travel extending from a negative throttle plate position through a zero or minimum throttle plate position at which air flow through the throttle valve is minimized to a maximum or wide-open-throttle position in which air flow is maximized. During operation of the electronic actuator, the throttle plate is operated between the minimum and maximum air flow positions. Inoperativeness of the actuator allows a biasing member to move the throttle plate to the negative throttle plate position assuring a default quantity of air flow to the engine and, therefore, continued engine operation. The negative position throttle body, referred to as an over-center design, involves costly manufacturing processes imposed by throttle bore/valve plate tolerances required to allow the throttle plate deflection through the zero or minimum air flow position.